Undercuts Are a Coverage Problem, Not a Resolution Problem

Orthodontic models usually scan “fine” at a glance—until you evaluate the areas that actually matter for ortho workflows: undercuts, embrasures, gingival margins, and interproximal anatomy. The typical failure is not that the scanner lacks detail; it’s that the workflow fails to capture the geometry from enough angles to make undercuts reliable.

At Repair.Dental, we approach this topic as coverage engineering: the job is to reduce shadowing, stabilise alignment, and prevent software from “guessing” missing undercut geometry that can later ruin appliance fit.

Dental insight: why undercut accuracy matters clinically

Dentists and orthodontists care about undercut capture because it directly affects:

aligner fit and retention
appliance seating (retainers, expanders, splints)
trim line accuracy and comfort
attachment planning and placement surfaces
predictable insertion/removal force (patient compliance)

If undercuts are under-captured or auto-filled, aligners may feel loose, appliances may bind, and chairside adjustments increase. Clinically, that becomes longer appointments and lower patient satisfaction.

Where undercuts hide (and why scanners miss them)

Undercuts tend to live in areas that are naturally difficult for line-of-sight capture:

deep interproximal embrasures
lingual surfaces near gingival margins
palatal rugae transitions and deep vault areas (upper)
crowded dentitions with tight contact zones
posterior segments, where curvature creates shadowed pockets

Even with good optics, scanners can’t “see around corners.” You must design the scanning approach to expose those surfaces.

Table: Undercut zone → scan risk → downstream orthodontic risk

Zone	Typical scan risk	Ortho consequence
Interproximal embrasures	shadowing, gap fill	loose aligner edges, poor retention
Lingual gingival margin	missing capture, smoothing	trim line discomfort, poor seating
Crowded anterior	occluded undercuts	attachment mismatch and fit issues
Posterior undercuts	incomplete lingual detail	appliance binding, chairside grinding
Palatal contours	holes or noise	inconsistent appliance adaptation

Angle strategy: undercuts require intentional capture paths

Ortho model scanning should not be one “standard” pass. Undercut capture improves when you plan a deliberate angle sequence:

Start with broad surface capture (baseline alignment)
Then add targeted angle passes to expose undercuts
Focus on areas where line-of-sight is poor: lingual and interproximal zones
Ensure the model is stable so those additional passes don’t introduce stitch drift

This approach reduces the common issue where a model looks complete, but undercuts are missing and auto-filled.

orthodontic model scanning on Repair Guru

Fixturing is critical: undercut passes amplify movement risk

Capturing undercuts usually requires more angles, and more angles mean more chances for micro-movement to ruin stitching. If the model shifts slightly, the software may average geometry and soften the very features you’re trying to preserve.

Stability controls that matter:

rigid, non-compressible mounting
consistent placement on the platform
balanced positioning to avoid wobble during rotation
no handling during capture

Ortho models are often larger and heavier; they can still rock if the base isn’t flat or the fixture is inconsistent.

Dental insight: patients feel undercut errors as comfort and compliance problems

Orthodontics has a patient compliance dimension. If appliances are uncomfortable, hard to seat, or feel unstable, compliance drops. That becomes a clinical outcome issue. Accurate undercut capture is therefore not just “technical accuracy”; it’s part of a predictable patient experience.

Surface quality: why gypsum texture and shine can mislead undercut capture

Undercut regions often have steep angles and small pockets. Surface noise or reflectivity can cause the scanner to “lose confidence” in those areas. Typical contributors include:

chalky stone dust sitting in embrasures
glossy sealants causing glare in steep angles
wet or contaminated surfaces, creating inconsistent reflectance
debris in crowded contacts

Before you blame the scanner, make sure the undercut pockets are clean and dry. Dirty undercuts produce false voids and patch fills.

Table: Symptom → likely cause → undercut-specific fix

Scan symptom	Likely cause	Fix focused on undercuts
“Filled-in” embrasures	insufficient angle coverage	add targeted interproximal passes
Holes on lingual gingiva	shadowing + reflectivity	reorient model + manage surface sheen
Jagged undercut edges	movement during multi-angle capture	improve fixturing stability
Softened anatomy	over-smoothing from noisy capture	clean debris + improve capture confidence
Inconsistent results	operator-dependent orientation	enforce a standard angle sequence SOP

Quality gate: verify undercut integrity before CAD

Ortho workflows often move fast, so undercut verification must be quick. A practical QC check is:

inspect key undercut zones (interproximal, lingual margins, crowded areas)
look for unnatural “smooth bridges” where gaps were auto-filled
compare left vs right symmetry to spot missing regions
confirm the model base and orientation match your SOP (repeatability matters)

This prevents the expensive scenario: designing and manufacturing aligners from compromised geometry.

Ortho-specific workflow differentiation: decide what level of undercut detail you need

Not every ortho use case requires the same undercut precision. A retainer may tolerate slightly less detail than a complex aligner case with attachments. Your lab should classify cases:

Standard ortho models: baseline undercut capture acceptable
Aligner/attachment cases: high-fidelity undercut capture required
Appliance-heavy cases: lingual and posterior undercuts become critical

This classification prevents overwork on low-risk cases and underwork on high-risk cases.

Dental insight: why clinicians sometimes blame the appliance, not the scan

When undercuts are under-captured, appliances may fit “almost” but require trimming or grinding. Clinicians may attribute this to manufacturing variation or design issues. The scan is often the root cause. If you can validate undercut capture as part of your QC, you protect both outcomes and accountability.

When to suspect equipment issues instead of technique

Undercut capture is demanding, but it should still be predictable with good technique. Consider equipment-side investigation when:

holes/noise increase across multiple case types, not only undercuts
repeatability tests fail on stable reference objects
capture confidence drops suddenly after updates or relocation
artefacts repeat in the same regions regardless of model orientation
calibration doesn’t hold or quality improves only briefly after recalibration

These patterns can indicate optics contamination, calibration instability, or mechanical issues affecting consistent capture.

How Repair.Dental isolates undercut problems quickly

When labs report “ortho models won’t scan undercuts,” we focus on isolating coverage vs equipment:

compare a standard scan pass to a targeted undercut angle sequence
evaluate whether missing regions align with predictable line-of-sight limitations
confirm fixturing and base stability
run a controlled reference scan to confirm scanner baseline

If the baseline is strong, the solution is SOP-driven. If baseline is weak, we escalate to diagnostics that restore stability and protect uptime.

Commercial takeaway: undercut capture is a competitive advantage in ortho

Orthodontic production is volume-driven, and small scanning errors scale into big rework costs. Labs that master undercut capture deliver aligners and appliances that seat predictably, require fewer chairside adjustments, and keep clinicians confident in the lab. Standardise angle coverage, stabilise fixturing, and verify undercut integrity—then undercuts stop being a pain point and start being a reliability feature.

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Orthodontic Model Scanning: Undercut Capture